The invention relates to loudspeakers and to low-cost magnetic motors for use in loudspeakers. The invention has application, among other places, in portable consumer electronics, in cell phones, pagers, digital music players, and other apparatus where weight and size are factors. It has particular utility in applications that rely upon a main power source having a relatively low voltage, e.g., between about three to approximately twelve volts, and in further aspects provides compact full range systems.
A large percentage of loudspeakers are electrodynamic speakers. Such speakers employ a magnetic driver to produce movement of a diaphragm (typically cone or dome-shaped sheet) which, in turn, causes sound. A typical loudspeaker includes a permanent magnet arranged to define a gap, and a voice coil positioned in the gap to which an audio-frequency signal is applied. The magnet may be mounted toward the rear of the frame, behind the diaphragm, and may utilize a magnetic circuit formed by one or more pole pieces arranged to define a high-flux gap, with the magnetic field focused or intensified in the gap. The voice coil is disposed adjacent the magnet, typically within the air gap, and may consist of conductive leads or wire formed about a cylindrical support or bobbin that is attached to the diaphragm.
In operation, electrical audio signals from an amplifier are applied to the voice coil producing a varying electromagnetic field around the coil which interacts with the magnetic field produced by the permanent magnet. The magnet is securely fixed to the frame and the voice coil is movable, so the voice coil moves as the two fields interact. Because the voice coil is coupled to the diaphragm via the support, its movement causes the diaphragm to vibrate. The vibration of the diaphragm causes air around the speaker to pressurize and depressurize producing sound waves in the air.
The high energy density of rare earth materials such as neodymium boron iron is attractive for creating and miniaturizing shielded loudspeaker magnets. The magnet rings or discs may be installed as cores on the inside of the voice coil for easy manufacturing, and the high fluxes allow high maximum levels of storable and extractable energy, so that such speakers may be efficiently driven.
However, the physics of sound generation, as well as the resistance or inductance of the coil tend to limit the frequency response and quality of sound achievable as the speaker size gets smaller. To some extent, one can compensate for non-linearities of response by compensating the gain of the drivers as a function of frequency. However, when one adds the constraint of using a low operating voltage, then the sharp drop in driving efficiency at the low end of the spectrum, and the increase in voice coil impedance at the high end, would seem to impose severe limitations on effectiveness of the technique of correction by drive power compensation.
Thus it would be desirable to provide improved small loudspeakers, with more uniform and/or extended response.
An object of this invention is to provide an improved loudspeaker and improved magnetic motor for a loudspeaker.
A further object of the invention is to provide a motor of low impedance and high engine efficiency for driving a loudspeaker.
A still further object is to provide motor that eliminates the need for multiple magnets and expensive edge winding and offers greater freedom in amplifier matching for best overall system value.
Still yet further objects of the invention are to provide such motors as permit the construction of low voltage sound systems for portable sound or voice appliances like cell phones, note book and palm size computers, pagers, and other interactive, wireless or computer audio appliances.